University of California, Riverside

Department of Electrical and Computer Engineering

Metal spintronics: Tunneling spectroscopy in junctions with magnetic...

Metal spintronics: Tunneling spectroscopy in junctions with magnetic...
Hyunsoo Yang
IBM Almaden Research Center, San Jose, CA

Date: Monday, March 20, 2006
Time: 11:00 am
Location: Bourns Hall A265

Recent advances in generating, manipulating and detecting spin-polarized electrons and their electrical current make possible entirely new classes of spin-based sensor, logic and storage devices. An important such device is the magnetic tunnel junction (MTJ) which has been under intensive study in recent years. Using conventional amorphous alumina tunnel barriers tunneling spin polarization (TSP) values of up to ~55% are found for conventional 3d ferromagnets, such as CoFe, but using highly textured crystalline MgO tunnel barriers TSP values of more than 90% can be achieved for otherwise the same ferromagnet. Such TSP values rival those previously observed only with half-metallic ferromagnets. Corresponding giant values of tunneling magnetoresistance (TMR) are found, exceeding 350% at room temperature and nearly 600% at 3K. The fundamental origin of tunneling magnetoresistance in MTJs is the spin-polarized tunneling current, which can be measured directly using superconducting tunneling spectroscopy (STS). The STS technique was first developed by Meservey and Tedrow using aluminum superconducting electrodes. Al has been widely used because of its low spin orbit scattering. However, measurements must be made at low temperatures (<0.4 K) because of the low superconducting transition temperature of Al. Here, we demonstrate that superconducting electrodes formed from NbN can be used to measure TSP at higher temperatures up to ~1.2K. The TMR is, however, depressed by insertion of ultra thin layers of both non-magnetic and magnetic metals in the middle of the MgO barrier. For ultra-thin, discontinuous magnetic layers of CoFe, we find evidence of Kondo assisted tunneling, from increased conductance at low temperatures (<50K) and bias voltage (<20 mV). Over the same temperature and bias voltage regimes the TMR is strongly depressed. We present other evidence of Kondo resonance including the logarithmic temperature dependence of the zero bias conductance peak. We infer the Kondo temperature from both the spectra width of this conductance peak as well as the temperature dependence of the TMR depression. The Kondo temperature is sensitive to the thickness of the inserted CoFe layer and decreases with increased CoFe thickness.

About the speaker:

Hyunsoo Yang earned his bachelors degree in electrical engineering from the Seoul National University in 1998 and M.S. and Ph.D. degrees in the electrical engineering department of Stanford University in 2003 and 2006, respectively. From 1988 he worked at a circuit and system design company, earning a patent award, until 2001, when he enrolled in Stanford's graduate school on a scholarship from the Korean Ministry of Information and Communications. He had been working on long-wavelength photonic devices using GaInNAs(Sb) for fiber communication channel at Stanford university. Since 2004, he has been at IBM-Stanford Spintronic Science and Applications Center. His doctoral research focused on the metal spintronics, especially magnetic tunnel junctions for the magnetic random access memory application. He was awarded the fellowship in the conference on Magnetism and Magnetic Materials for 2005 and the American Physical Society (GMAG) outstanding dissertation award for 2006.
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